Lithium-ion secondary batteries are common in portable consumer electronics because of their high energy-to-weight ratios, lack of memory effect, and slow self-discharge when not in use. Rechargeable lithium-ion batteries are also being designed and manufactured for use in automotive applications to provide energy for electric motors to drive vehicle wheels.
The basic unit of a lithium-ion battery is an individual cell which includes an anode, a cathode and a liquid, non-aqueous electrolyte suitable for carrying and conveying lithium ions. Lithium-ion batteries of different sizes, shapes and electrical capabilities may be fabricated by electrically connecting any suitable number of these cells in parallel, series or a combination of these to develop a battery of suitable voltage and capacity. Within an individual cell, the anode on discharge becomes the cathode on charge, and the cathode on discharge becomes the anode on charge. From here forward, the electrode that is the anode on discharge (the negative electrode) will be referred to as the anode, and, correspondingly, the electrode that is the cathode on discharge (the positive electrode) will be referred to as the cathode.
In the anode, elemental lithium is often stored between the sheets or layers of a graphite structure forming lithium-intercalated graphite. During discharge, lithium ions migrate out of the lithium-graphite while, during charge, the lithium ions are re-inserted into the graphite. The cathode may be formed from any lithium based active material that can sufficiently undergo lithium intercalation and deintercalation. For example, in various embodiments, cathode may comprise, among others, at least one of spinel lithium manganese oxide (LiMn2O4), lithium cobalt oxide (LiCoO2) and nickel-manganese-cobalt oxide [Li(NixMnyCoz)O2].
A lithium-ion battery generally operates by reversibly transporting lithium ions between its negative and positive electrodes. To prevent physical contact (electron-conducting contact) between the anode and cathode which would result in an internal short circuit, a separator, is positioned between the electrodes. The separator, commonly a polyolefin polymer is microporous and contains small pores which are filled with electrolyte to provide pathways for passage of lithium ions from one electrode to the other. Microporous separator materials in common use include polyethylene or polypropylene. The microporous separators may be about twenty-five to about thirty microns thick and exhibit thirty-five percent or more porosity.
Each of the negative and positive electrodes is also carried on or connected to a metallic current collector (typically copper for the anode and aluminum for the cathode). During battery usage, lithium is oxidized at the anode to form lithium ions which are then transported from the anode and received by the cathode, passing through the ion-conducting electrolyte in the separator pores to form an internal circuit. The current collectors associated with the two electrodes are connected by a controllable and interruptible external circuit which allows an electron current to pass between the electrodes. The external electron current serves to electrically balance the internal circuit resulting from the transport of lithium ions through each cell.
The battery may then be re-charged by passing a suitable direct electrical current in the opposite direction between the electrodes. During recharging, the flow of lithium ions is reversed and they pass from cathode to anode where they are reduced to lithium metal and re-intercalated into the graphite.
In principle, such a discharge-recharge procedure may be practiced indefinitely. But, under normal operating conditions, battery life is affected by the degradation of the active materials (e.g. anode, cathode and electrolyte) and abnormal operation can induce the formation of lithium dendrites, surface deposits of lithium on the anode. These dendrites, with continued growth may penetrate the thin polymer separator and to enable a direct connection, a short circuit, between anode and cathode. Also, if any fine metal particles are introduced into the inter electrode space during manufacture these too may enable a short circuit.
Penetration of commonly-used polymer microporous separator materials, such as polyethylene or polypropylene, is more likely at more elevated cell temperatures. For example at cell temperatures of greater than 130° C. the separator materials will soften appreciably and offer reduced resistance to penetration. Even if penetration of the separator does not occur, any prolonged exposure to temperatures in excess of 130° C. may result in shrinkage or even melting of the separator. Clearly, any of these behaviors, shrinkage, softening or melting, will diminish the separators ability to provide electrical insulation between the battery anode and cathode to prevent internal short circuits.
There is thus a need for a more durable and temperature tolerant microporous separator for lithium-ion battery cells.